1. Field of the Invention
This invention relates to a potential sensor (electric-potential sensor) with which the voltage of a detection object is measured, and to an image forming apparatus having the potential sensor.
2. Description of the Related Art
A potential sensor disclosed in Japanese Patent Application Laid-open No. H07-056414 detects the surface potential of a photosensitive member charged by a charger in a digital color copying machine. In this potential sensor, the surface potential of the photosensitive member charged by setting the grid voltage of the charger at VG1 is detected, and the value detected is represented by VS1. The surface potential of the photosensitive member charged by setting the grid voltage of the charger at VG2 (provided that the voltage has the relationship of VG1<VG2) is also detected, and the value detected is represented by VS2. Then, on the basis of an absolute value ΔVS of the difference between the VS1 and the VS2 and an absolute value ΔVS′ of the difference between VS1′ and VS2′ detected under an ideal environmental condition, the potential sensor corrects its sensitivity.
A potential sensor disclosed in Japanese Patent Application Laid-open No. H07-013396 also detects the surface potential of a photosensitive member in an image forming apparatus. This potential sensor is provided with an abnormality detector which has the following means to detect abnormal conditions of the potential sensor. That is, the abnormality detector has a means for applying at least two types of voltages to the surface whose potential is to be measured with the potential sensor, a means for causing the potential sensor to measure several times the potential of the surface, a means for calculating an average value from measured values, and a means for calculating from the measured values a maximum value and a minimum value or a difference between the two. It further has a means for judging whether or not the average value based on the two types of voltages is within a certain range, and a means for judging whether or not the maximum value and the minimum value or the difference between the two are within certain values, to thereby detect the abnormal conditions of the potential sensor by the above judging means.
In general, in the technical field of potential sensors, stains on the potential sensor are given as the cause of detection of abnormal conditions. The cause of such stains includes toner used in copying machines, or foreign matter such as paper dust and oil mist. In the techniques described in the above background art, the abnormal conditions of the sensor due to toner or the like having adhered to the sensor are managed by taking measures of, e.g., correcting sensitivity or providing an abnormality detecting means.
However, in both cases, the correction of sensitivity or the detection of abnormal conditions must be performed when the potential sensor is in use in the copying machine, during which normal potential measurement cannot be made for the potential measurement object in the copying machine. In addition, image forming apparatus is unusable because of the detection and correction of abnormal conditions.